Although lipoprotein lipase acts on chylomicrons and VLDL at the luminal surface of capillaries, the enzyme is synthesized in parenchymal, not endothelial, cells, Mice born with combined lipase deficiency (cld/cld) have mostly inactive lipoprotein lipase in hear and brown adipose tissue, and are unable to clear triacylglycerol from the blood. Heart of normal mice had lipoprotein lipase, visualized with fluorescent immunochemistry, associated with small blood vessels, whereas heart of cld/cld mice had lipoprotein lipase within myocytes, but not in blood vessels. Brown adipocytes cultured from normal mice had lipoprotein lipase only on cell surfaces, whereas adipocytes from cld/cld mice contained lipoprotein lipase within the cells. These findings suggest that secretion of lipoprotein lipase is impaired in myocytes and adipocytes of cld/cld mice. Active lipoprotein lipase is present in liver of both normal and cld/cld mice. Fluorescent immunochemical studies in tissues from both groups of mice showed that the enzyme was located in hepatocytes. Non-esterifed cholesterol has been localized in cultured human fibroblasts incubated with low density lipoproteins. Cholesterol was visualized with fluorescent antibodies to cholesterol and with filipin fluorescence. Lysosomes were also localized, with fluorescent antibodies to lysosomal membrane protein. Intracellular co-localization of the fluorescent probes demonstrates accumulation of cholesterol in lysosomes of fibroblasts in culture.